Numerous constructions of control members of the above described type are known, whether the latter relate to a push rod, corrector or winder. It is common to all these constructions that the annular packing, generally constituted by an O-ring seal, is compressed radially between the tube and the centre of the head (see for example Swiss Pat. No. 581 340) or between the tube and an auxiliary ring (see French Pat. No. 2 235 414). Hitherto, such an arrangement has always seemed necessary for ensuring the seal between the head and the tube.
The introduction of increasingly thinner watches has caused the problem of manufacturing control members, in particular push rods or correctors which can be fitted on very shallow cases. The problem particularly occurs of manufacturing a push rod having a tube whereof the diameter is less than 2 mm and more precisely a diameter of 1.70 mm. On constructing a push rod according to the prior art, which satisfied the standards relating to water-tightness, it was found that the push rod was unusable owing to the frictional forces of the packing, which frictional forces become equal to or greater than the force of a return spring. The use of a more powerful spring which was theoretically possible, although giving rise to another problem in view of the extremely small space for housing the latter and thus limiting the diameter of the wire of the spring to a dimension of the order of 0.15 mm, proves in practice to be a useless solution for two reasons. The first is the danger of separation of the head from the control rod by spring pressure, the head and control rod usually being force fitted together. The second is the excessive difficulty the user experiences in view of the very small surface area of the head and the painful pressure exerted on the user's finger when the user attempts to actuate the push rod by pressing on the head. Admittedly, lubrication of the packing is able to reduce the friction, but only constitutes a palliative whereof the effect lasts for a short duration. Finally, it has been proved that it is virtually impossible to force fit known control members of the aforesaid type into the watch case without this resulting in a certain buckling, i.e. misalignment of the head and the rod with respect to the tube. The consequence of this misalignment may be the separation of the packing from the head or tube, the effect of which is to reduce or even destroy the effectiveness of the seal when the packing is exclusively compressed radially between the tube and head. This drawback could be eliminated by increasing the compression of the packing, but the effect of this measure is to further increase the frictional forces. In French Pat. No. 2 235 414, it was proposed to remedy this drawback by fitting the annular packing between the tube and a cylindrical ring which was free radially in the head, the packing being compressed radially between the tube and the ring. However, this solution requires an additional part and considerable hollowing-out of the head and its fitting is delicate.